Mark VanGessel, Extension Weed Specialist; firstname.lastname@example.org
I have been asked a number of questions about postemergence spraying for soybeans. First and foremost, weed height should 3-inches or less for maximum control. We have all heard the stories about how an application of a certain product “cleaned up” a field of big/tall weeds. For every situation where someone controlled large weeds, there are multiple fields where the same large weeds were not controlled and they produced seeds.
At this point of the season, marestail or horseweed is large and should be treated as soon as possible with glufosinate (Liberty), or where its appropriate, dicamba (Engenia/Xtendimax/Tavium) or Enlist One or Enlist Duo. Glyphosate will cause some suppression of marestail and maybe the only option for varieties with only the Roundup Ready trait.
Common ragweed that are resistant to glyphosate, group 2 herbicides (such as FirstRate), and group 14 herbicides (such as Reflex or Cobra) have few options. Glufosinate (Liberty), or where its appropriate dicamba (Engenia/Xtendimax/Tavium) or Enlist One or Enlist Duo are the best options for most fields. Basagran is another option for small plants.
Palmer amaranth plants in this area are often resistant to glyphosate and group 2 herbicides, so herbicide options are limited to fomesafen (Reflex), Blazer Ultra, glufosinate (Liberty), or where its appropriate dicamba (Engenia/Xtendimax/Tavium) or Enlist One or Enlist Duo. Because Palmer amaranth has a prolonged germination period, be sure to include a residual herbicide with the postemergence application. Options include metolachlor/s-metolachlor (such as Dual Magnum), acetochlor (Warrant), pyroxasulfone (such as Zidua SC or Anthem Maxx), or Reflex.
Fields infested with Palmer amaranth and resistant-common ragweed should receive top priority for postemergence applications.